Method and system for measuring patterned structures

ABSTRACT

A method and system are presented for determining a line profile in a patterned structure, aimed at controlling a process of manufacture of the structure. The patterned structure comprises a plurality of different layers, the pattern in the structure being formed by patterned regions and un-patterned regions. At least first and second measurements are carried out, each utilizing illumination of the structure with a broad wavelengths band of incident light directed on the structure at a certain angle of incidence, detection of spectral characteristics of light returned from the structure, and generation of measured data representative thereof. The measured data obtained with the first measurement is analyzed, and at least one parameter of the structure is thereby determined. Then, this determined parameter is utilized, while analyzing the measured data obtained with the second measurements enabling the determination of the profile of the structure.

FIELD OF THE INVENTION

This invention is in the field of measurement techniques, and relates toan optical system and method for the accurate measurement of parametersof regular patterned structures. The invention is particularly useful incontrolling a lithography process.

BACKGROUND OF THE INVENTION

Lithography is widely used in various industrial applications, includingthe manufacture of integrated circuits, flat panel displays,micro-electro-mechanical systems, micro-optical systems etc. Generallyspeaking, the lithography process is used for producing a patternedstructure. During the manufacture of integrated circuits, asemiconductor wafer undergoes a sequence of lithography-etching steps toproduce a plurality of spaced-apart stacks, each formed by a pluralityof different layers having different optical properties. Eachlithography procedure applied to the wafer results in the pattern on theuppermost layer formed by a plurality of spaced-apart photoresistregions.

To assure the performance of the manufactured products, the applicationsof the kind specified above require accurate control of the dimensionsof the sub-micron features of the obtained pattern. When dealing withwafers, the most frequently used dimensions are the layer thickness andthe so-called “critical dimension” (CD). CD is the smallest transversedimension of the developed photoresist, usually the width of the finestlines and spaces between these lines. Since the topography of themeasured features is rarely an ideal square, additional informationfound in the height profile, such as slopes, curves etc., may also bevaluable in order to improve the control of the fabrication process.

Generally, an ordinary optical microscope can be used for measuringfeatures' dimensions. A microscope is practically capable of measuringline width with a resolution of no less than 0.1 μm. The currenthigh-performance semiconductor devices, however, have features'dimensions of 0.18 μm, and require CD measurement with, the resolutionof a few nanometers.

Several Optical CD (OCD) measurement techniques recently developed relyon imaging a certain test pattern which is placed in a special test areaof the wafer. These techniques utilize various methods aimed atamplifying tiny differences in the line-width to obtain macroscopiceffects that could be resolved by visible light, although the originaldifferences are more than two orders of magnitude below the wavelengthused. However, some of these techniques do not rely on fundamentalphysical effects, and thus could be more effective in some cases andless effective in others.

Another kind of technique utilizes scatterometric measurements, i.e.,measurements of the characteristics of light scattered by the sample. Tothis end, a test pattern in the form of a grating is usually placed inthe scribe line between the dies. The measurement includes theillumination of the grating with a beam of incident light anddetermining the diffraction efficiency of the grating under variousconditions. The diffraction efficiency is a complicated function of thegrating line profile and of the measurement conditions, such as thewavelength, the angle of incidence, the polarization and the diffractionorder. Thus, it is possible to gather a wealth of data thereby allowingthe extraction of information about the line profile.

Techniques that utilize the principles of scatterometry and are aimed atthe characterization of three-dimensional grating structures and thedetermination of line profiles have been disclosed in numerouspublications. Publications, in which diffraction efficiency was measuredversus wavelength, include, for example the following:

(1) A. Roger and D. Maystre, J. Opt. Soc. Am, 70 (12), pp. 1483-1495(1979) and A. Roger and D. Maystre, Optica Acta, 26 (4), pp. 447-460(1979) describe and systematically analyze the problem of reconstructionof the line profile of a grating from its diffraction properties (theinverse scattering problem). A later article “Grating ProfileReconstruction by an Inverse Scattering Method”, A. Roger and M.Breidne, Optics Comm., 35 (3), pp. 299-302 (1980) discloses how the ideadisclosed in the above articles can be experimentally used. Theexperimental results show that the line profile can be fitted such, thatthe calculated diffraction efficiency will closely match the diffractionefficiency measured as a function of wavelength for “−1” diffractionorder. The comparison of these experimental results with electronmicroscopy measurement showed a reasonable agreement.

(2) “Reconstruction of the Profile of Gold Wire Gratings: A comparisonof Different Methods”, H. Lochbihler et. al., Optik, 98 (1), pp. 21-25(1994) deals with the comparison of the results of several experimentaltechniques. Both optical transmittance and reflectance efficiencies weremeasured in the “0” order as a function of wavelength. By fitting themeasurements to theoretical spectra calculated using diffraction theory,the grating profile was found. Comparison of these results with theresults of X-ray diffraction efficiency and electron microscopy showed agood agreement.

(3) Voskovtsova, L. M. et al., Soviet Journal of Optical Technology 60(9) pp. 617-19 (1993) studies the properties of gratings fabricated byreplica technique. It has been found that the line profile of thehologram diffraction grating differs from the calculated sinusoidalprofile. This difference leads to a difference in the spectraldiffraction efficiency, an effect that was utilized for process control.

(4) Savitskii, G. M. and Golubenko, I. V., Optics and Spectroscopy 59(2), pp. 251-4 (1985) describes a theory for the reflection propertiesof diffraction gratings with a groove profile which is a trapezoid withrounded corners. Such gratings can be fabricated by a holographictechnique with photosensitive materials. It was found that theparameters of the trapezoidal profile, such as the depth of the groove,the width of a flat top and the slope of the side walls, affect thediffraction efficiency of the grating working in the auto collimationregime for the “−1” order.

(5) Spikhaltskii A. A., Opt Commun 57 (6) pp. 375-379 (1986) presentsthe analysis of the spectral characteristics of gratings etched into adielectric material. It has been found that these characteristics can besignificantly varied by slightly changing the grating groove profile.

(6) U.S. Pat. No. 5,867,276 discloses a technique for broadbandscatterometry, consisting of the illumination of a sample with anincident light beam having a broad spectral composition and detecting abeam of light diffracted from the sample with a spectrometer. Thetechnique is aimed at obtaining the spectrally-resolved diffractioncharacteristics of the sample for determining the parameters of thesample. The patent suffers from the following drawbacks: themeasurements are done in the “0” diffraction order which is insensitiveto asymmetries in the profile; and the analysis is done using the NeuralNetwork (N.N.) method, which is sub-optimal by nature for applicationsrequiring a high resolution. Additionally, the method does not take intoaccount the need to focus the light onto a small spot, which isdetermined by the small area of the test structure allowed in the scribeline.

According to another group of publications, a monochromatic light source(e.g. laser) is utilized, and grating profile parameters are extractedfrom the measurement of the diffraction efficiency versus incidenceangle. Such publications include, for example the following:

(A) S. S. H. Naqvi et al., J. Opt. Soc. Am. A, 11 (9), 2485-2493 (1994)discloses a technique that utilizes measurement of the diffractionefficiency in “0” order versus incidence angle to find the height ofetched grating. Calculations are based on the Rigorous Coupled WaveTheory (RCWT), initially developed by Moharam and Gaylord and disclosedin M. G. Moharam and T. K. Gaylord, J. Opt. Soc. Am, 71, pp. 811-818(1981), and several existing statistical techniques for the fittingstage.

(B) Raymond, J. R. et al., SPIE 3050, pp. 476-486 (1997) discloses atechnique that utilizes a laser beam scanning with a range of angles tomeasure the diffraction efficiency versus incidence angle and to extractthe line profile from the measured data.

(C) U.S. Pat. Nos. 4,710,642 and 5,164,790 disclose optical instrumentswhich require to rotate the sample under test, which is definitely adisadvantage.

(D) U.S. Pat. Nos. 4,999,014; 5,889,593 and 5,703,692 discloseinstruments employing angle-dependent intensity measurements without therequirement to rotate the sample. According to these techniques,different optical arrangements are used for providing the changes of theangle of incidence of an illuminating monochromatic beam onto the sample(wafer), without moving the sample. According to U.S. Pat. No.5,703,692, the measurement is carried out by mechanically scanning theangle of incidence using a rotating block. The main disadvantages ofsuch a technique are as follows: it requires the use of moving parts,the calibration of an angle in a dynamical situation, and has a limitedangle range which does not provide enough information allowing accurateextraction of profile. According to U.S. Pat. No. 5,889,593, an opticalarrangement includes a first lens that serves for focusing incidentlight onto a wafer at a range of angles, and a second lens that servesfor focusing diffracted light onto a detector array. Although thistechnique does not need any moving parts, since the measurements aresimultaneous, special care has to be taken to destroy coherence andavoid interference between the different light paths. Any suitablecomponent for destroying the coherence always reduces the systemresolution, thereby reducing the amount of obtained information.

In a third group of publications, the diffraction efficiency is measuredwhen both wavelength and incidence angle are constant. In this case,information is extracted from the comparison of diffraction efficiencyof several orders. This group of publications includes, for example, thefollowing documents:

(I) U.S. Pat. No. 4,330,213 discloses a line-width measurement systemusing a diffraction grating. In this system, the intensities of firstand second order light components are obtained to determine theline-width using empirical formulae.

(II) U.S. Pat. No. 5,361,137 discloses another example of the use of aconventional scatterrometry technique. Here, a set of intensities of the“1” or “2” diffraction order image of the set of “fixed-line width andvariable-pitch-width” test gratings is recorded. From this set ofintensities, line-width can be calculated.

Generally speaking, the conventional techniques use the followingmethodology in order to analyze the measured results:

First, a model is assumed for the grating profile having a number ofparameters that uniquely define the profile. The user defines therequired model (type of model) and sets the limits and the requiredresolution for each of the desired parameters.

Second, a spectral library is prepared using an optical model. Thespectral library contains the calculated spectra for all possibleprofiles as defined by the user.

Third, given a measured spectrum, a fitting procedure finds the profilewhose calculated spectrum included in the spectral library best matchesthe measured spectrum.

SUMMARY OF THE INVENTION

There is accordingly a need in the art to facilitate the control of themanufacture of patterned structures by providing a novel method andsystem for measurements in a patterned structure to determine a lineprofile of the structure, utilizing the principles of scatterometry.

The term “patterned structure” signifies a structure comprising aplurality of spaced-apart stacks (elements) each including differentlayers, the pattern being formed by patterned regions and un-patternedregions. The term “pattern region” used herein signifies is a regionincluding elements (stacks) having different optical properties, and theterm “unpatterned region” signifies a region with substantially uniformoptical properties, as compared to the patterned region. Such anun-patterned region is comprised of a single stack including differentlayers having different optical properties.

The main idea of the present invention is based on obtaining measureddata from at least two measurements applied to the same patternedstructure (e.g., wafer) in order to achieve both high accuracy and highreliability measurements. The entire measurement procedure is carriedout is several steps, taking a different measurement at each step.Analysis, likewise, is performed in several steps, wherein each analysisstep utilizes the information obtained in the previous steps. The twomeasurements could be applied at two different measurement siteslocated, respectively, in patterned and un-patterned regions. The twomeasurements may be carried out so as to detect light returned from thestructure with different solid angles of propagation, or with differentstates of polarization.

According to the present invention, at least one parameter of theprofile considered in an optical model used for measurements isdetermined by analyzing at least one preliminary measurement applied toa predetermined site on the structure (wafer). The preliminarymeasurement is inherently different from further measurements by eitherthe type of site under measurements or the measurement conditions(angle, polarization, wavelength range, diffraction order, etc.). Forexample, the preliminary measurement utilizes normal incidence of anilluminating beam, while the further measurement utilizes obliqueillumination. Data (parameters) obtained through this preliminarymeasurement is used for optimizing the fitting procedure, therebyimproving further measurements applied to other locations on thestructure.

Preferably, the parameters obtained through the preliminary measurementinclude the reflectivity and thickness of at least one layer underneaththe uppermost layer. Additionally, the at least one preliminarymeasurement allows for determining optical constants (i.e., refractionand absorption coefficients n and k) and thickness of the regions of theuppermost layer.

There is thus provided according to one aspect of the present invention,a method of determining a line profile in a patterned structure forcontrolling a process of manufacture thereof, wherein the patternedstructure comprises a plurality of different layers, the pattern in thestructure being formed by patterned regions and un-patterned regions,the method comprising the steps of:

-   -   carrying out at least first and second measurements, each of the        measurements utilizing illumination of the structure with a        broad wavelengths band of incident light which is directed on        the structure at a certain angle of incidence, detection of        spectral characteristics of light returned from the structure,        and generation of measured data representative thereof;    -   analyzing the measured data obtained with the first measurement        and determining at least one parameter of the structure; and    -   analyzing the measured data obtained with the second measurement        and utilizing said at least one parameter for determining the        profile of the structure.

According to another aspect of the present invention, there is provideda measurement system for determining a line profile in a patternedstructure comprising a plurality of different layers, the pattern in thestructure being formed by patterned regions and un-patterned regions,the system comprising a measuring unit including an illuminationassembly and a collection-detection assembly, and a control unit coupledto output of the measuring unit, wherein:

-   -   the illumination assembly produces incident light of        substantially broad wavelengths band directed onto the structure        at a certain angle of incidence, and the collection-detection        assembly detects spectral characteristics of light returned from        the structure and generates measured data representative        thereof;    -   the measuring unit is operable for carrying out at least first        and second measurements and generating measured data        representative of the detected returned light; and    -   said control unit is operable to be responsive to the generated        measured data for analyzing the measured data obtained with the        first measurement to determine at least one parameter of the        structure, and utilizing the at least one determined parameter        while analyzing the measured data obtained with the second        measurement for determining the line profile of the structure.

The scatterometry based measurement technique provides the collection ofa large amount of data from each measured profile, e.g., the diffractionefficiency in a large number of different angles or a large number ofwavelengths. This richness of data may allow the fitting of themeasurements to the results of a multi-parameter model describing themeasured profile, thus providing more information than merely statingthe CD. This additional information also provides confidence in theresults, particularly if the effective number of independent measuredvalues is significantly larger than the number of free parameters in themodel. Since exact models describing diffraction from general profilesand in general situations have been developed for years and are known tobe of high accuracy, these methods have a good chance of obtainingaccurate results.

The system according to the invention can be applied as an integratedmetrology tool. In contrast to all conventionally used off-linemeasurement tools, occupying a large footprint and requiring additionalmanual operations that slow down the entire fabrication process andallow only the measurement of samples from each production lot, thesystem of the present invention may be integrated as part of theproduction machine, thus allowing full automation of the manufacturingprocess. For this integration to be possible, the system should be veryeconomical in space.

Additionally, the operation of the system is fast enough, so that everysemiconductor wafer in the production line can be measured, allowingcloser control over the process. The system of the present inventionenables a multi-stage measurement procedure, thereby improving thequality of the entire measurement. The measurement technique accordingto the invention requires only a small measurement site in accordancewith the area constraints, which characterize current lithography.

More specifically, the present invention is used for process control inthe manufacture of semiconductor devices (wafers), e.g., the control ofa lithography process, and is therefore described below with respect tothis application.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 is a schematic illustration of a wafer structure;

FIGS. 2A and 28 are schematic illustrations of two possible examples,respectively of the line profile showing some parameters thereof to bemeasured;

FIG. 3 is a schematic illustration of the main components of ameasurement system constructed according to one embodiment of theinvention;

FIG. 4 is a schematic illustration of the main components of ameasurement system constructed according to another embodiment of theinvention;

FIG. 5 is a schematic illustration of the main components of ameasurement system constructed according to yet another embodiment ofthe invention;

FIG. 6 is a schematic illustration of one more embodiment of theinvention;

FIG. 7 is an example of a part of a production line utilizing the systemof either of FIGS. 3, 4, 5 and 6;

FIG. 8 illustrates another example of a production line utilizing thesystem of either of FIGS. 3, 4, 5 and 6; and

FIG. 9 is a schematic illustration of a system utilizing severalmeasurement systems according to the invention using a common serverutility.

FIG. 10 is a flowchart illustrating the method according to the presentinvention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, there is illustrated (not in a correct scale) awafer W that typically has a plurality of stacks formed by differentlayers, and presents a structure with a periodic pattern. Measurementsare aimed at determining the profile of the periodic pattern (“grating”)formed in one or more of the wafer's layers—layer 2 in the presentexample. Generally, the periodic pattern may involve more than a singlelayer (which are not specifically shown here), provided that theperiodicity in all patterned layers is equivalent. This periodicity maybe either one-dimensional (i.e. repeated lines) or two-dimensionalperiodicity (i.e. finite-area units repeatedly placed on the nodes of atwo-dimensional grid). The patterned layer is enclosed between aplurality of un-patterned, underlying layers 3 a and a plurality ofun-patterned, over-lying layers 3 b terminated by a background medium 3c (e.g. air).

The patterned layer 2 may include both patterned and un-patterned sites.In the un-patterned site, e.g. L₁, L₃, a single material, either M_(a)or M_(b), is to be found within an area larger than the area of themeasurement spot. As to the patterned site, e.g. L₂, both materialsM_(a) and M_(b), having different optical constants, are to berepeatedly found within the area of the measurement spot. In specificapplications, materials M_(a) and M_(b) may take different identifies.For example, both the materials of all over-lying layers and one of thematerials M_(a) and M_(b) are identical to the background medium, .e.,the measured pattern is a relief pattern. Such a relief pattern could beformed, for example, by a post-developed photoresist, or by apost-etched Poly-Silicon, Aluminum or Silicon dioxide, either stackedwith photoresist or not. Examples in which none of the materials M_(a)and M_(b) is equivalent to the background medium may includepost-exposure (undeveloped) photoresist, or “dual-damascene” type Copperlines. An example of two-dimensional periodicity would be atwo-dimensional array of via-holes formed as a test pattern in order tomeasure the diameter and other parameters of the via-hole process.

Usually, the smallest transverse dimension of the pattern is called the“critical dimension” (CD), however other definitions of the CD may bealso applied. Usually, the CD of the developed photoresist determinesthe CD of the patterns formed in later stages of the entiremanufacturing process, thus bearing extra importance.

The model of the periodic structure will depend on the specificapplication and on the requirements of the end-user. For example, FIG.2A illustrates a graph G₁ exemplifying a possible model for lineprofile, being trapezoidal with rounded corners. In this case, theparameters to be determined may include the following:

-   -   height H of the profile G (i.e., the thickness of the        photoresist);    -   critical dimensions CD_(B) and CD_(T) at the bottom and top of        the photoresist region, respectively;    -   radius R_(B) and R_(T) of the curvatures at the bottom and top        of the photoresist region, respectively; and    -   the period of grating.

The above is the example of a symmetrical line profile. In the case ofan asymmetrical profile, an additional tilt angle should also bedetermined. More elaborate models can also be used, e.g., by dividingthe profile into several layers, each layer described by a geometricshape (e.g., trapeze), while requiring matching of the profile width inthe interfaces between layers. This is exemplified in FIG. 2B showing atwo-part trapezoidal line profile G₂ formed by top and bottom trapezoidsG^((B)) ₂ and G^((T)) ₂. In this case, the parameters to be determinedmay, for example, include the following:

-   -   total height H_(tot) of an envelope E defined by the profile G₂        (i.e., the thickness of the photoresist);    -   the average value of the critical dimension CD_(aver)        corresponding to the width of the envelope E at a height H₀        equal to the half of the total height H_(tot);    -   radius R of the curvature at the top of the photoresist region;    -   the tilt of the envelope a with respect to the horizontal plane        P;    -   the maximal distance t between the profile G₂ and the envelope        defined thereby;    -   the height H_(b) of the bottom-part trapezoid G^((B)) ₂; and    -   the period of grating.

Measurement is based on obtaining the diffraction efficiency spectrumfrom a grating on the wafer. The grating is any periodic structure inone or two dimensions composed of features whose parameters should bemeasured, e.g. line-width, through holes diameters, etc. Due to theperiodic structure, the diffraction from the features on the wafer islimited to a discrete number of angles (diffraction orders), as governedby the diffraction equation:

$\begin{matrix}{{\sin \; \Theta_{r}} = {{\sin \; \Theta_{i}} + {n\frac{\lambda}{d}}}} & (1)\end{matrix}$

where Θ_(i) is the incidence angle, Θ_(r) is the reflected angle, λ isthe wavelength, d is the grating period and n is the order number (n=0being the specular reflection).

It should be noted that the measured gratings could be either anintegral part of the operative portion of the wafer or a test pattern.Such small test structures which are typically smaller than 40 μm×40 μmare measured using a focusing optics.

Reference is made to FIG. 3, illustrating a measurement system 1constructed and operated according to the invention for measuringparameters of a wafer W (constituting a patterned structure). The system1 includes a measurement unit 4, a support stage 5 for supporting thewafer W and a control unit 6. Also provided in the system 1, is a waferhandler, which is not specifically shown. The wafer handler serves forloading/unloading wafers to and from the stage 5, and may include asuction means for holding the wafer. Generally speaking, the waferhandler and wafer stage serve together for receiving wafers from aprocessing tool (not shown here), pre-aligning them along coordinateaxes (e.g., by rotating the handler), maintaining, placing in ameasuring position and returning them to the same or another processingtool.

The measurement unit 4 defines two measurement channels, generally at 8a and 8 b, respectively. Each measurement channel includes such mainconstructional parts as illumination and collection-detectionassemblies.

The illumination assembly of the channel 8 a is composed of a lightsource 10, for example a Xenon arc lamp, a controlled polarizer 11, abeam splitter 12 and an objective lens 14 that is driven by a suitablemotor (not shown) for auto-focusing purposes. The light source 10generates incident light B⁽¹⁾ _(i) of a broad wavelength band. Thepolarizer 11 serves to separate only light components of the desiredpolarization and allow its collection. The beam splitter 12 serves forspatially separating incident and returned light components. It shouldbe noted that the polarizer could be accommodated in any point along theoptical path. However, in order to avoid possible changes in thepolarization state of light induced by optical elements located betweenthe polarizer and the wafer, the polarizer is preferably positioned asclose as possible to the wafer. The collection-detection assembly of thechannel 8 a includes a spectrophotometric detector 16 and a beamsplitter 17 in the form of a pinhole mirror, the purpose of which willbe explained further below. As shown, an additional polarizer 15 a maybe accommodated in the optical path of light ensuing from the pinholemirror 17 and propagating towards the detector 16. In the measurementchannel 8 a, the incident light B⁽¹⁾ _(i) normally impinges onto thewafer W, and light B⁽¹⁾ _(r), specularly reflected (normal “0” order),is collected and directed towards the detector 16, in a manner describedfurther below.

The illumination assembly of the measurement channel 8 b is composed ofa light source 18 (e.g., the same Xenon arc lamp) generating incidentlight B⁽²⁾ _(i) of a broad wavelength band, an objective lens 20 whichis similarly associated with a suitable drive (not shown) forauto-focusing purposes, and a controlled polarizer 21. Thecollection-detection assembly of the channel 8 b includes a collectinglens 22, a spectrophotometric detector 24 and, optionally, a controlledpolarizer 23 (shown in dashed lines). The incident light B⁽²⁾ _(i)impinges onto the wafer W at a certain angle (e.g., 60°), and aspecularly reflected light component B⁽²⁾ _(r) (oblique “0” order) isdetected. It should be noted that alternatively, either one of thepolarizers 21 and 23 or both of them could be used in the measurementchannel 8 b. The provision of the polarizer, included in either one ofillumination and collection-detection channels, or both is associatedwith the fact that diffraction efficiency is also a function ofpolarization, resulting in inherently different diffraction efficiencyspectra in “perpendicular” polarizations. Additionally, the appropriateselection of light polarization may provide better sensitivity toparameters of the line profile. Assume, for example, that the so-called“conventional mounting” of the illumination assembly, namely such thatthe oblique incident beam B⁽²⁾ _(i) propagates towards the wafer in aplane perpendicular to the lines of the grating. If a dielectric patternin measured (e.g., post-developed photoresist, post-etch silicon orpost-etch oxide), TE-polarization is preferred, i.e., the vector of theelectric field is perpendicular to the plane of incidence and parallelto the grating lines. If post-etching measurement is performed on ametallic patterned structure, TM-polarization is preferred, i.e., thevector of the electric field lies in the plane of incidence and isperpendicular to the grating lines.

It should be noted, although not specifically shown, that optical fibersmay be used for directing light components ensuing from the pinholemirror 17 and lens 22 to the detectors 16 and 24, respectively. Hence,the detectors could be mounted at any suitable location. A suitabledrive assembly is provided for moving the optical elements in the X-Y,thereby enabling the measurements at different locations on the wafer.Additionally, the wafer stage 5 is also equipped with a drive assembly(not shown), which allows for aligning the wafer along the Z-axis,rotating the wafer in the horizontal plane, and leveling the waferaround two rotational axes such that the surface of the wafer will beparallel to the X-Y plane of the optical elements. The requirement forthe leveling is derived from the sensitivity of the measurements to theangle of incidence. The requirement for rotating the wafer in thehorizontal plane (so-called “e-control”) is derived from the fact thatthe diffraction depends on the mounting direction. Thus, in order tobring the wafer to the so-called “conventional mounting” position, thisdegree of freedom is required. Additionally, this degree of freedomallows for using a window that covers only a half of the wafer. Hence,the two halves of the wafer are sequentially measured by rotating thewafer by 180° with respect to the window. This concept is described in aco-pending application assigned to the assignee of the presentapplication. Such a technique enables to save considerable foot printarea which is a critical factor when using a measurement system as anintegrated metrology tool.

It should also be noted that the system 1 could be provided with adynamic auto-focusing assembly enabling high-speed measurements.Auto-focusing could be performed either with each measurement channelseparately, by moving one or more of its optical elements (lenses), orwith both measurement channels, using the wafer stage Z-axis control.

As further shown in FIG. 3, an additional, imaging channel 26 isprovided. Channel 26 includes the illumination assembly of the “normalincidence” channel 8 a, a polarizer 15 b and an imaging detector 30(e.g., a CCD camera) receiving light components reflected from thepinhole mirror 17.

The polarizers 15 a and 15 b are used in the collection-detectionassemblies of the measurement channel 8 a and imaging channel 26.Alternatively, a single polarizer 13 (shown in dashed lines) couldsubstitute the polarizers 11, 15 a and 15 b. While, the option of usingthe single polarizer 13 is more economical in price and space ascompared to the option of using three polarizers 11, 15 a and 15 b, theuse of three separate polarizers allows for measuring in the preferredpolarization whilst observing the wafer surface with the imaging channelthrough perpendicular polarization. This configuration, called“Nomarski”, is similar to dark-field configuration in the sense that thepattern edges are strongly enhanced in the image, thus allowing forbetter, more accurate pattern recognition in some cases. The polarizer15 b in the imaging channel 26 could thus be mounted for rotation so asto change its preferred polarization, or for shifting between its twooperational positions so as to be in or out of the optical path, peruser's choice.

The main principles of the construction and operation of a measurementsystem including the zero-order detection spectrophotometer (measurementchannel 8 a) and the imaging channel 26 is disclosed in U.S. patentapplication Ser. No. 08/497,382, assigned to the assignee of the presentapplication. This document is therefore incorporated herein by referencewith respect to this specific example.

The pinhole mirror 17 separates a central part B⁽¹⁾ _(r1) (about 20 μm)of light B⁽¹⁾ _(r) specularly reflected from the illuminated spot andcollected by the lens 14, and allows its propagation towards thespectrophotometric detector 16. A periphery part B⁽¹⁾ _(r2) of lightB⁽¹⁾ _(r) is reflected from the mirror 17 towards the imaging detector30. As a result, the measurement area, considered in thespectrophotometric detector 16, presents a “dark” central region,typically 40 μm in diameter, in the center of the field of view of theimaging channel, typically being 20 mm×20 mm, both measured on thewafer. This approach enables to locate the measurement area in theentire illuminated spot defined by the field of view of the CCD.

The outputs of the spectrophotometric detectors 16 and 24 and theimaging detector 30 are coupled to the control unit 6. The control unit6 is typically a computer device having a memory for storing referencedata (libraries), one or more processors for analyzing data coming fromthe detectors and controlling all the operations of the measurementsystem 1 including driver(s), light sources, power supply, interface,etc. The preparation of libraries will be described further below. Thecontrol unit 6 also displays the measurement results. The processor isoperated by suitable image processing and pattern recognition software,capable of both global and site-to-site alignment. The alignmenttechnique based on the features of the pattern is disclosed in U.S. Pat.Nos. 5,682,242 and 5,867,590, both assigned to the assignee of thepresent application. Thus, the control unit 6 is capable of locating andprocessing measurements. The analysis of the measured data could be usedfor establishing feedback closed-loop control of a correspondingprocessing tool, as will be described further below.

Another feature of the present invention consists of the optional use ofa second spectrophotometer for calibrating the light sources' spectra.It is known that several types of light sources have spectralcharacteristics varying in time. Thus, any previous measurement(calibration) of the incident spectrum will lead to significant errorsin interpretation. By taking a known fraction of the incident light(e.g., in channel 8 a, the signal reflected by the beam splitter) andmeasuring its spectral characteristics simultaneously with themeasurement of the diffraction signal, this problem can be avoided.Alternatively, a photodiode whose output has been previously learnedcould be used to calibrate for intensity variations, assuming therelative spectrum to be constant.

Reference is made to FIG. 4 illustrating a measurement system 100,constructed and operated according to another embodiment of theinvention. To facilitate understanding, same reference numbers are usedfor identifying those components which are identical in the system 1 and100. In the system 100, a measurement channel 34 collecting a lightcomponent of “−1” diffraction order is provided instead of the oblique“0” order measurement channel 8 b. The illumination assembly of themeasurement channel 34 includes the light source 18, the polarizer 21and the objective lens 20, while its collection-detection assemblyincludes a linear detector array 36, for example, a plurality ofphotodiodes and an optical light guiding assembly 37 (e.g., fiberbundle, set of prisms or combination of both). Here, each of the lensesis of a small numerical aperture (NA=0.1) providing the angles of lightpropagation within a range of ±5°. In this specific example, the angleof incidence of the light component B⁽²⁾ _(i) is about 60°, and lightcomponents B⁽³⁾ _(r), being the “−1” order of diffraction from the waferpropagate within a large solid angle, e.g., 50°, and are collected bythe linear detector array 36. The optionally provided light-guidingassembly 37 serves for transferring the light diffracted in the “−1”order from the vicinity of the wafer W to a remote location of thedetector array 36, thus allowing some flexibility in its location.

The measurement channel 34, in distinction to the channel 8 b, does notneed a spectrophotometer. Indeed, the photoresist grating alreadydisperses the light diffracted in the “−1” order, and therefore there isno need for any additional dispersive element in the system. However, itis also possible to use the light-guiding assembly for focusing lightcollected at the “−1” order to the entrance of a spectrophotometer, thusconverging and re-dispersing the optical signal. This configurationtogether with an appropriate switching device (not shown) will eliminatethe use of the special detector array 36 for detecting the “−1” order byusing the same spectrophotometer 16 for sequential measurement of both“0” and “−1” orders. The use of one or more switching devices of thesame kind would allow the utilization of a single spectrophotometer forall the measurement channels 8 a, 8 b and 34.

It should be noted that if the resulting signal is not sufficientlystrong, a cylindrical lens (anamorphic optics) could be used toconcentrate more energy onto the detector. Additionally, in order todetect the spatial distribution of different wavelengths within thesolid angle corresponding to the “−1” diffraction order, an appropriatecalibration procedure is previously carried out, for example, by usingspectral filters, or by finding spectral peaks of the light source usinga calibration target having sufficiently smooth diffraction efficiencyin a spectral region around the peaks.

Turning back to the diffraction equation presented above, theconfiguration of FIG. 4 allows for collecting, for example, thefollowing wavelength ranges: (a) in the case that d=0.36 μm, 344nm<λ<606 nm, or (b) in the case that d=0.26 μm, 248 nm<λ<438 nm, bothcases correspond to Θ_(r)=(−5°)−(−55°).

The above wavelength ranges are, on the one hand, sufficiently wide(i.e., λ_(max)/λ_(min)=1.76) and substantially outside the DUV region,and, on the other hand, approach the following condition: λ=d. Thisallows for meaningful information to be gained.

FIG. 5 illustrates a measurement system 200 which includes threemeasurement channels—channel 8 a (normal “0” order), channel 8 b(oblique “0” order) and channel 34 (“−1” order), and the imaging channel26. For simplicity, the different polarizers are not shown here. Thisconfiguration combines the measurement channels of the systems 1 and100, thus increasing the amount of obtainable information.

Referring now to FIG. 6, there is illustrated a measurement system 300,where the collection-detection means of the normal “0” order” and “−1”order form a unified collection channel 8′, utilizing a common objectivelens 14′ and a common spectrophotometric detector 16. To this end, bothlens 14′ and lens 20′ have a larger numerical aperture, as compared totheir counterparts in the previously described systems. An aperture 38is provided, being mounted for movement along the lens 14′ in a planeparallel thereto. Each location of the aperture 38 (locations I, II andIII in the present example) “opens” only a small part of lens 14′. Thus,at each position of the aperture 38, only light components propagatingwithin a solid angle defined by the aperture, and therefore being only asmall part of the entire solid angle covered by lens 14′, are directedtowards the detector 16. Since the diffraction angle Θ_(r) in non-zeroorders is determined by both the incidence angle and the wavelength, andthrough the diffraction equation (1) above, fixing the diffraction anglemeans that the incident angle is a function of the wavelength. For afixed collection angle Θ_(r) and n=−1, one thus has:

$\begin{matrix}{\Theta_{i} = {\arcsin \lbrack {{\sin \; \Theta_{r}} + \frac{\lambda}{d}} \rbrack}} & (2)\end{matrix}$

The obtained spectrum is thus inherently different from those obtainedby prior art techniques, since, in distinction to these techniques,neither the angle of incidence nor the wavelength is fixed. Themeasurement system according to the present invention allows for amultiplicity of diffraction angles Θ_(r), and for increasing the amountof information that can be collected, as well as the accuracy andreliability of the results.

As to the normal “0” order, it is successfully collected when theaperture 38 is shifted so as to be centered with the optical axis of thelens 14′ (position III), and illumination from the light source 10 isused instead of that of the light source 18.

Reference is made to FIG. 7, showing a part of the wafers' productionline utilizing the measurement system of the present invention. Here,the measurement system, for example the system 1, is associated with alithography arrangement 40. This arrangement 40 typically includescoater, exposure and developer tools T₁, T₂ and T₃, loading andunloading cassette stations C₁ and C₂, and a robot R. The constructionand operation of such a lithography arrangement are known per se, andtherefore need not be specifically described.

In the present example, the system 1 is integrated with the arrangement40, and is accommodated in a manner allowing its application to a waferensuing from the developer tool T₃. The control unit of the inventedsystem is coupled to a control unit of the exposure tool T₂ (not shown)for feedback purposes, for example for adjusting the exposure dose/time,focusing, etc. It should, however, be noted that the system 1 could alsobe coupled to the coater and/or developer tool for adjusting theirparameters (e.g., photoresist thickness, post-exposure baking time,development time, etc.) prior to processing the next coming wafer. Asfor the measured wafer, it can be returned for reprocessing, if needed.

It should also be noted that data indicative of the wafer's profile canbe used for adjusting the parameters of an etching tool prior to itsapplication to the measured wafer or the next coming wafer, i.e., forfeed-forward purposes. Alternatively or additionally, the measurementsystem can be used for post-etching measurement.

FIG. 8 shows another example of a production line utilizing themeasurement system of the present invention, for example the system 1.Here, the measurement system is integrated into a complete CD controlsystem 500. The system 500 includes the following components:

-   -   a system 60 for measuring the parameters of the photoresist and        of the under-lying layers prior to exposure, which system is        composed of a measuring unit 61 and a control unit 62;    -   the system 1 for measuring CD and other profile parameters,        which system comprises the measuring unit 4 and the control unit        6;    -   a set of controllers 71, 72, 73 for controlling tools T₁, T₂,        T₃, respectively, of the lithography arrangement; and    -   a system controller 80.

The main idea of the system 500 is that information from bothpre-exposure and post-develop stages is combined, allowing for acomplete closed-loop-control utilizing both feedback and feed forward.The operation of the system 500 is mainly guided by the controller 80,which receives information from both systems 60 and 1. An expert system,which is a learning software tool running on the controller 80,accumulates the information from different measurements (across thewafer, wafer to wafer, etc.) and learns the correlation betweenpre-exposure and post-develop measurement. The expert system also learnsthe effectiveness of using different control parameters related to toolsT₁, T₂, T₃, in the reduction of variations in the resulting CD and themethods to combine them in the most effective way.

One of the features, that the system 1 (or similarly systems 100, 200,300) will require in order to be effectively integrated into the system500, is OCR (optical character recognition) capability for identifyingeach wafer by its identity number. Identifying the wafers in adeterministic way is important for the reliability of the systemcontroller 80, as well as for the integration of system 500 withpost-etch measurements carried out on other processing tools in the fabthrough a common communication network or database. OCR capability couldbe achieved either by scanning the area containing the identity numberusing the X-Y stage and the viewing channel 26, or, alternatively, by aspecial OCR channel (which is not specifically shown). Such an OCRchannel may include a CCD camera, imaging optics, and a controllerrunning OCR software.

The operation of the measurement system according to the invention willnow be described. Setup of the measurement includes the following twostages:

1) Definition by the user of a profile model to be used and ranges foreach parameter of the selected model. Additionally, knowledge about allthe layers in the wafer and their optical properties, and any additionalrelevant information concerning the product (wafer) to be measuredand/or the measurement conditions, is desired for defining themeasurement sites.

2) Preparation of a library of spectra (reference data) corresponding toall the possible profiles. Each spectrum in the library provides thediffraction efficiency for a given profile of the grating, givenpolarization, given angle of incidence and mounting method, givennumerical aperture of the system, and a given diffraction order as afunction of wavelength.

An important feature of the present invention refers to the fact thatone has to take into account the finite numerical aperture of thesystem. This numerical aperture, required for measuring small sites,means that light is incident on the wafer at a considerable range ofangles at the same time. Since diffraction efficiency is a sensitivefunction of the incident angle, failing to consider this fact willresult in significant error. In order to take this effect intoconsideration, one can calculate the diffraction efficiency from eachprofile at several angles of incidence around the central one (i.e., theaverage direction of the solid angle of light propagation). Then, theweighted average of diffraction efficiency spectra at the differentangles will be calculated in order to obtain the effective diffractionefficiency for the entire cone (solid angle) of angles of incidence. Forexample, in a system having the central angle of incidence equal to 60°,the diffraction efficiency corresponding to 57°, 60° and 63° angles ofincidence could be calculated, and then weighted with respective weightsof 0.25, 0.5 and 0.25. The fact that a small number (e.g., three) ofsuch angles is sufficient to describe a continuum of angles is nottrivial, and the selection of the number, spacing and averaging of thedifferent angles may be application dependent.

The calculation is made using the known Rigorous Couple Wave Analysismethod, modal methods, or by a hybrid method containing parts of bothprevious methods.

The preparation of the library may be made in one or more stages. Forexample, the following scheme may be used:

(1) Initially, the spectra corresponding to a small number of profilesonly are calculated, sparsely sampling the whole multi-dimensional spaceof possible profiles.

(2) At this point, several measurements are taken and analyzed using theinitial library. Approximate average values of the desired parametersare then determined, describing an approximate average profile of thegrating.

(3) A sub-space of possible profiles is defined around the averageprofile. The sub-space is sampled with the required (final) resolution,and the spectra of all profiles in the sub-space are calculated.Alternatively, the spectra in the sub-space can be calculated “uponrequest”, i.e., when required for the interpretation of consecutivemeasurements.

(4) The rest of the profile space is divided into sub-spaces withincreasing distance in the parameter space from the average measurement.

(5) These sub-spaces are consecutively sampled and their correspondingspectra are calculated until the whole parameter space is calculated inthe final resolution.

The advantage of the above scheme is that the continuous measurement canstart already after step (3) thereby significantly saving the setuptime. In fact, the above-described dynamic process of the librarypreparation allows to operate the system almost immediately after therecipe is prepared. Initially, the system will support only a reducedthroughput, since it will have to rely mainly on slow, real-timespectrum calculations. At this stage, only some of the wafers will bemeasured on-line, while others will be measured, but their results willeither be stored for later interpretation (if needed) or interpretedwith a very low accuracy, using the crude initial library. With time,the system will support higher and higher throughput, since most of therelevant parts of the spectral library will be ready, until the maximalthroughput is obtained when the whole library is prepared.

Additionally, in distinction to alternative techniques, in which thesystem has no independent ability to prepare a library on-site, theabove dynamic scheme advantageously has the possibility of handlingvariations in optical constants. It is well possible that over time, theoptical constants of some layers will change slightly. This change couldresult from lot-to-lot variations due to the changes of the propertiesof photoresist (e.g. composition) or slight changes in processconditions (e.g. temperature, humidity). The chemical producer maydisregard such changes since they are not supposed to have any directeffect on the process (e.g. changes in the optical constants ofphotoresist at wavelengths different from the exposure wavelength). Onthe other hand, any change in the optical constants of the measuredlayers will obviously have an effect on the measurement with themeasurement system (1, 100, 200, 300). In order to avoid this problem,the system has to monitor on a continuous basis the optical constants ofthe layers, and, in case that these deviate significantly from theconstants used for the calculation of the library, the library has to berebuilt. If the changes in the optical constants are sufficientlysmooth, a system with on-board computational power will be able tofollow the changes without a significant deterioration in themeasurement accuracy. Obviously, any technique that relies on externalcomputational power will be disadvantageous in the scenario.

As an alternative to geometrical profile models (e.g. trapeze), theprofiles to be used for the preparation of the spectral library could beobtained through simulation of the relevant process. This is in contrastto the above technique of producing possible profiles that has noa-priori connection to the real process. For example, if the patternedlayer is developed photoresist, than simulation of the lithographicprocess may be used to obtain expected line profiles from inputparameters such as resist thickness, absorption coefficient, sub-layerreflection, exposure wavelength and doze, and parameters of the exposuresystem, such as numerical aperture and focus conditions. In this case,each set of such input parameters will result in a correspondingexpected profile, and the preparation of the spectral library willinclude an additional step. The required steps thus are as follows:

(a) The user defines the type of process, type of model and the requiredinput parameters, describing the situation prior to the process and therange of possible parameters for the process;

(b) Simulation of the process is used to produce a large set of possibleprofiles according to some or all possible combinations of processparameters and uncertain parameters of the structure prior to theprocess;

(c) An optical model is used to produce the expected spectrum for eachprofile according to experimental conditions of the measurement(incidence angle, numerical aperture etc.).

Step (c) of this preparation process (and the following fitting process)does not depend on the way the initial profiles have been prepared. Aclear advantage of using a process-related method for profilepreparation is that a greater variety of inherently different profilescan be used, thus the chances of finding the real profile are increased.Additionally, by providing the input parameters used for the simulationof the profile in addition to the actual profile, the system may providemore ready-to-use information to the user, and may possibly indicate thesource of deviations found in the process.

Measurement Procedure

Step 1. Alignment of the wafer 2 is performed by the wafer handler andwafer stage, so as to provide the correct position and orientation ofthe wafer with respect to the measurement system 1. Alignment iscontrolled by feedback from position and angle sensors typicallyprovided in the measurement system, as well as from the imaging channel26. The alignment procedure is a very important stage of the entiremeasurement process, since diffraction efficiency is also a function ofthe angles between the incidence beam, normal to the wafer's surface andthe direction of the grating.

Step 2. The first measurement site is found. This is implemented byproviding a relative displacement between the objective lens (andpossibly other optical elements) and the wafer along two mutuallyperpendicular axes within a plane parallel to the wafer's surface. Forthis purpose, feedback from images of some parts of the wafer acquiredby the CCD camera 30 can be used.

Step 3. The measurement of reflection coefficient spectrum is carriedout with the measurement channel 8 a (“normal incidence “0” order”)applied to one or more so-called “unpatterned site”. Turning back toFIG. 1, such sites L₁, L₃ are located where there is no lateralvariation within an area larger than that of the measurement spot of themeasurement channel. These measurements enable to determine thethickness, reflectivity and optical constants (refraction index andextinction coefficient, n and k) of one or more layers including thepatterned layer 2.

Step 4. The relative location between the wafer 2 and the incident beamB⁽¹⁾ _(i) is changed (e.g., by moving either the support stage or theoptics of the measurement system) to a further measurement site L₂having a required grating structure (a patterned site, as describedabove). Measurement of the reflection efficiency spectra is carried outwith the “normal incidence” measurement channel 8 a, in one or morepolarization states. These measurement can be later utilized to extractparameters such as the thickness of the patterned layer 2, gratingparameters, and optionally also optical constants.

It should be noted that generally, steps 3 and 4 could in some cases becombined, namely the above parameters of one or more underneath layerscould also be determined at step 4, whilst measuring in the photoresistregion by the measurement spot less than the dimensions of this region.In other words, the determination of the parameters of the patternedlayer and those of one or more underneath layers could be carried out atsuch measurement site(s) and with such measuring conditions, that thespectral characteristic of light returned from the measurement spot isnot significantly affected by the line profile.

It should, however, be noted, that, if the optical constants of apatterned layer are known or could be considered to be stable for somebunch of wafers (lot), the “normal incidence” measurement channel andthe “oblique incidence” measurement channel could be applied to the samesite(s) L₂. The measurements are preferably separated in time. Such atechnique is time saving, since it eliminates the need for additionalmovements from the “unpatterned” site to the “patterned” site.

Step 5. Measurement of the reflection efficiency spectra is carried outat the oblique incidence in one or more diffraction orders (i.e., “0”and/or “−1” order), with the measurement channels 8 b and/or 34,respectively, or in the case of system 300 through the unifiedmeasurement channel 8′. These measurements are applied to patternedsites L₂ as defined above (FIG. 1). Measurements can be taken from oneor more grating structures per measured die, where different gratingsmay have different line/space ratios in order to simulate differentconditions of the controlled process.

The Analysis of Measured Data

Step A: Initially, the normal incidence measurements (steps 3-4, or step4 only) are analyzed to extract the above parameters of one or moreunderneath layers and of the photoresist layer, and to determine thethickness and optionally optical constants of the photoresist layer.Some of these measurements, e.g., yielding the optical constants n and kcan be carried out only once per several measurements of step 5 above,namely once per wafer, once per lot, etc.

Step B: The spectral characteristics measured with either one of the“oblique incidence” channel 8 b (step 5) and the “normal incidence”channel 8 a (step 4), or both are compared to the correspondingreference data, i.e., spectra stored in the library, and a so-called“best-fitting” between the measured and reference data is found. In thisstep, the results of the previous step are used to limit the scope ofthe search, thus reducing computation time. Having found a sufficientlygood fitting for all the spectra, one can conclude that the measuredstructure has a profile most similar to that with which the“best-fitting” spectra have been determined, and can output theparameters of this profile.

By carrying out the analysis in the above two steps A and B, the problemof finding the best fitting profile is significantly reduced. Thisreduction is gained by de-coupling some of the parameters, e.g.,heights, thus reducing the number of independent parameters in step Babove. Since finding an optimum in a multi-parameter space is a problemwhose complexity considerably increases with the number of independentparameters (dimensions), the de-coupling results in a faster, moreaccurate and more stable solution.

Several algorithms are required for the interpretation of the measureddata (after the spectral library is prepared). Analysis of the layer(s)'thickness is based on algorithms utilized in NovaScan System(commercially available from Nova Measuring Instruments Ltd.) and mayalso apply other algorithms, for example an analysis of the opticalproperties of the semiconductor layer(s) utilizing a technique disclosedin U.S. Pat. No. 4,905,170 with some modifications. The fitting of themeasured data to the reference data (i.e., spectral library) utilizesknown statistical multivariate techniques such as Neural Networks,genetic algorithms, etc.

In a FAB for the wafers' manufacture, several concurrently operatedproduction lines are usually utilized, which perform either the same ordifferent manufacturing steps. Consequently, several measurement systemsconstructed and operated according to the invention could be installedwithin these production lines. In this case, the control units of thedifferent measuring systems can be associated with a local area network(LAN), with a common server utility installed outside the productionline, and possibly remote from the FAB being connected to the LANthrough the computer network, e.g., the Internet. It is also possiblethat a common server utility is associated with control units ofdifferent measurement systems installed at different FABs.

FIG. 9 illustrates a measurement system 400 based on the above concept.The system 400 is composed of several measuring modules, eachconstructed similarly to either one of the systems 1, 100, 200 and 300(system 1 being shown in the present example). Thus, each such measuringmodule 1 comprises the measuring unit 4, applied to the waferprogressing on a corresponding part of the production line.

As illustrated in FIG. 9, the system 400 includes a server utility 42and several measuring modules, e.g. system 1, connectable to the server42 through a communication link 44. The server 42 is a central processorof the entire system 400, and may perform different tasks at differentoperational steps. The communication network serves for connecting theserver 42 to the measuring modules 1, providing the connection betweenthe measuring modules 1, as well as connection between the server 42 anda host machine (not shown) of the FAB to enable the closed loop controlof a corresponding processing tool.

During the setup of measurement, the server 42 is responsible forreceiving information from the user and preparing reference data(libraries of possible models). The reference data is then transmittedover the communication network to the corresponding measuring module 1.

During the measurement procedure, the server 42 may perform thefollowing tasks:

1) Monitor the operation of the measuring module 1 in the system 400.This utilizes both the receiving of signals from sensors, that monitorinternal parameters of the measuring module (e.g. temperature, lightsource parameters, etc.), and generating an alarm signal in case ofmalfunction or evidence for required preventive maintenance.

2) Display measurement results of any one of the measuring modules tothe operator. The displayed information may include the statisticalanalysis of any sub-group of results (e.g., in-wafer statistics,wafer-to-wafer statistics, lot-to-lot statistics, module-to-modulestatistics, etc.).

3) Perform all or part of the interpretation of the measured data. Forexample, the machine-specific calculations, such as normalization tocalibration data, could be made in each measuring module, while thecomparison between measured spectrum and reference data could be carriedout at the server.

During the maintenance procedure, the server may display the sensors'output, perform the remote control of various mechanisms and provideon-line assistance (“help interface”).

The preparation of the reference data (libraries) may be carried out byany suitable technique, for example as follows:

Preparation by the server 42 only;

Preparation by each control unit 6 separately, as described above withrespect to the operation of the system 1;

Parallel preparation by several control units associated with differentmeasuring modules connected to the same network;

Preparation by a distant server connected to the local server 42 ordirectly to the control units 6 through the network, e.g. the Internet.

The advantages of the present invention are thus self-evident.Measurement is carried out in both normal and oblique “0” orders, andmay be measured both in “0” and “−1” orders. Thus, a wealth of data ismeasured which can result in more accurate reconstruction of theprofile. Since the “−1” order light is that diffracted by the samplegrating, it is possible to place a detector array and measure thediffracted signal directly as it comes from the wafer.

The analysis is carried out in several steps, using first the mostsensitive measurement channel to measure each parameter before finaloptimization, and is done using data from all channels. This methodreduces the possibility of finding a local minimum of the fittingfunction, which is not the correct profile.

The analysis is carried out using a Genetic Algorithm or anothertechnique that does not depend on a training stage, in which the systemlearns from calibration examples. This increases the chances for correctmeasurement, and reduces the setup time.

The system allows for the measurement on small test structures(typically smaller than 40 μm×40 μm) utilizing focusing optics. The factthat a significant angle range is used is taken into account in thecomputation to avoid misinterpretation. The system is designed in a waythat will enable its incorporation into a closed loop control system forcontrolling CD.

Those skilled in the art will readily appreciate that variousmodifications and changes can be applied to the embodiments of theinvention as hereinbefore exemplified without departing from its scopeas defined in and by the appended claims.

1. A method of generating a library of simulated-diffraction signals ofa patterned structure, said method comprising: obtaining at least onesimulated profile by simulation of a manufacturing process of apatterned structure; associating hypothetical process parameters with asimulated profile; varying the hypothetical process parameters within arange to generate a set of hypothetical profiles; adjusting the range tovary the hypothetical parameters based on the process simulation;determining the required hypothetical process parameters to associatewith the hypothetical profile based on the process simulation; andgenerating a set of simulated signals from the set of hypotheticalprofiles based on an optical model.
 2. The method of claim 1 comprising:determining a resolution for the set of simulated signals; and varyingthe hypothetical process parameters used in generating the simulatedsignals at an increment corresponding to the determined resolution. 3.The method of claim 1, wherein said set of simulated signals isgenerated using the adjusted range to vary the hypothetical parameters.4. The method of claim 1 comprising: verifying the quality of thegenerated set of simulated signals.
 5. The method of claim 1 comprising:generating a library by pairing each simulated signal in the set ofsimulated signals with each hypothetical profile in the set ofhypothetical profiles; and storing said pairings of simulated signalsand hypothetical profiles.